The invention pertains generally to catalysts and in particular to encapsulated cure catalysts.
Thermoplastic polymers cure via one or more chemical reactions whose rate determines both the pot life and the cure time. Consequently, a lengthy pot life results in a lengthy cure time, thereby necessitating increased heating and a long overall processing time. If a cure catalyst is added, then the pot life shortens and serious processing problems can arise.
One application for thermoplastic polymers where this dilemma is causing unacceptable difficulties is PBX explosives (plastic bonded explosives) which comprise a thermoplastic binder and one or more high-energy materials. In contrast to melt-cast explosive systems which solidify upon casting and cooling, the PBX explosives require a large amount of curing space or holding area.
Encapsulation has been used to delay the reactivity or activity of one or more ingredients in energetic compositions. Glass microspheres are used to isolate one ingredient of a Sprengel explosive in U.S. Pat. No. 3,797,392. The initiation of the detonation cracks the glass microspheres and allows the entire explosive to detonate. In U.S. Pat. No. 3,507,719 and 3,388,554 by James E. Hodgson, particles of a metal fuel are encapsulated in order to hold them in a stable suspension in solid propellants. A liquid explosive is sensitized, in U.S. Pat. No. 3,713,915 by C. R. Fast, by the addition, just prior to use, of a sensitizing agent encapsulated with cellulose acetate, cellulose triacetate, polyvinyl chloride or polyvinyl acetate. The encapsulation material is soluble in nitromethane, causing the release of the sensitizing agent in a short time after the addition. The curing agent and liquid prepolymer for a flare composition are encapsulated with a crushable material in U.S. Pat. No. 3,728,172 by Dillehay et al. The ingredients are put into the flare container and a rupturing pressure is applied, causing the liquid prepolymer to polymerize and forming the flare.
To date, encapsulation has not been successfully used to improve the processing of plastic bonded explosives. The encapsulation material must be inert with the encapsulated material but it must also be either soluble in or reactive with one or more ingredients in the PBX explosive. Of course the degree of solubility or reactivity is equally important because the encapsulated ingredient must be released at the correct time, neither too soon nor too late.